Why Does My Aircon Leak Only When It Rains?

Every wall-mounted aircon installation requires a penetration through the exterior wall — the pipe sleeve through which the refrigerant lines, drain pipe, and electrical cable pass. When this penetration is not properly sealed with waterproof compound, or when the sealant degrades after a few years of sun and rain exposure, rainwater can enter the gap and track along the pipe sleeve into the interior wall cavity. From there it follows gravity to the lowest accessible point — often appearing at the trunking edge, the indoor unit frame, or a wall seam well away from the original entry point.

The misdiagnosis in this case is almost always drain blockage, because the water appears near the indoor unit and the location looks like a condensate leak. The critical difference is that a drain fault produces dripping during unit operation regardless of weather, while rain ingress only produces water during or immediately after rainfall — even if the aircon is off. Failing to trace the water to its entry point before clearing the drain or flushing the condensate line means the ingress path stays open and the leak returns with the next storm.

• –Leak appears during rain and reduces after weather clears.
• –Water trails follow wall opening or trunking route.
• –Cooling may still feel normal.

Rain ingress through a wall opening produces leaking that tracks directly with rainfall — the unit can be completely switched off and the drip still appears. Unlike drain backflow, which requires the unit to be running and a specific wind-pressure pattern at the outlet, this path is purely weather-driven and repeatable on any rain event. Unlike the electrical risk path, which escalates quickly and involves breaker trips or odors, rain ingress here typically shows as clean water along a wall seam or trunking edge. Trace the trail back toward the wall penetration and check whether the pipe sleeve sealant is intact. We trace the entry path from outside, seal the wall penetration or trunking gap, and confirm the indoor leak stops after the next rain event. Gas or cooling work will not fix a rain ingress path.

The condensate drain pipe exits to the exterior of the building at a point exposed to outdoor air. During heavy rain accompanied by strong wind — a common pattern in Singapore's squall-line storms — sustained pressure builds at the drain outlet and can push air, and in severe cases condensate water, back through the pipe. The backflow collects at the lowest indoor point, which is usually the drain tray. Once the tray fills, it overflows through the same path as a standard drain blockage.

What makes this difficult to diagnose is that it only happens under specific weather conditions — not every rainstorm triggers it, only those with sustained wind from the direction the drain outlet faces. A technician who visits during dry weather, finds the drain clear, and concludes there is no fault will send the homeowner back to the same recurring problem. The fix involves changing the drain outlet orientation, adding a wind-guard or check valve, or re-routing the discharge point to a sheltered location — none of which are identified without understanding the weather dependency.

• –Leak appears while unit runs during rain periods.
• –Water pattern changes with wind and rain intensity.
• –Drip points shift between outlet and nearby paths.

Drain backflow only occurs when the unit is actively running during rain accompanied by strong wind — it requires both conditions simultaneously. Unlike direct rain ingress, which happens whether the unit is on or off, backflow is operation-dependent and tied to specific squall-line weather patterns rather than every rainstorm. Unlike the electrical risk path, there is no breaker trip or burning smell. The drip comes from the same path as a regular drain leak, which is why weather dependency is the critical diagnostic question: note whether the leak stops immediately when the unit is switched off during a storm. We inspect the drain outlet exposure and routing, add a wind-guard or re-route the discharge point if back-pressure is confirmed, and retest during operation. Treating this as a permanent indoor leak without weather context can mislead diagnosis.

When rainwater enters through a wall penetration or tracks along the pipe sleeve, it does not always take the most visible path. It can run inside the trunking, along the wiring conduit, or between the wall lining and the structure before becoming visible. The aircon isolator switch, terminal block inside the indoor unit, and the PCB wiring harness are all potential interception points — and any of them represents a serious shock and fire risk if water contacts live terminals under load.

The danger is that the electrical symptom — a breaker trip, a flickering display, or a burning smell — appears to be an aircon fault rather than a water ingress problem. Technicians focused on the cooling system may inspect the PCB or compressor while the actual cause is rainwater sitting in the wiring conduit above. This scenario requires a combined electrical safety inspection and ingress trace before any unit is switched back on. Operating a unit with wet wiring — even briefly to test whether it trips — is not an acceptable diagnostic step.

• –Water appears near switches, isolator, or wiring points.
• –Breaker trips during rain-linked leak events.
• –Electrical smell appears with leak pattern.

This path is distinguished from both ingress and drain backflow paths by the involvement of electrical symptoms — a breaker that trips during rain, a burning or plastic smell near the indoor unit, or visible moisture at the isolator switch or wiring conduit. Unlike the other two paths where the water is cosmetically damaging but not immediately dangerous, here rainwater has reached live components. The unit can appear to operate normally between rain events, which is what makes this the most easily underestimated of the three patterns. Stop using the unit until electrical safety is confirmed. We isolate ingress and verify safe power condition first. Turn the circuit breaker off and do not attempt a restart. Do not operate the unit while water is near electrical points — even briefly to test whether it trips again.